Materials which are used for electrical or electronic parts as electric current carrying parts such as connectors, lead frames, relays, and switches are not only required to have good “electrical conductivity” for the purpose of suppressing the generation of Joule heat due to electric current conduction but required to have high “strength” for withstanding a stress given at the time of assembling or operation of an electrical or electronic appliance. In addition, electrical or electronic parts such as connectors are also required to have excellent bending workability because they are in general formed by bending work after stamping.
In particular, in recent years, in electrical or electronic parts such as connectors, downsizing and weight reduction tend to advance. Following this, in sheet materials of a copper alloy as a base material, a requirement for thinning (for example, a sheet thickness is not more than 0.15 mm, and moreover not more than 0.10 mm) is increasing. For that reason, a strength level and an electrical conductivity level required in the base material become much stricter. Specifically, base materials having not only a strength level such that the 0.2% yield strength is 950 MPa or more but an electrical conductivity level in which the electrical conductivity is 30% IACS or more are desired.
In addition, in electrical or electronic parts such as connectors, a “factor of bending deflection” is used at the time of designing because they are in general formed by bending work after stamping. The factor of bending deflection means an elastic modulus at the time of a bending test, and when the factor of bending deflection is lower, it is possible to increase the amount of bending deflection until the permanent deformation is started. In particular, in recent years, in order to respond to not only the design to permit a scattering in sheet thickness or residual stress of the base material but a need to attach importance to an “inserting feeling” of a terminal portion in practical use, a structure which undergoes large spring displacement is demanded. For that reason, in mechanical properties of the base material, it is advantageous that the factor of bending deflection in the rolling direction is small as not more than 95 GPa, and preferably not more than 90 GPa.
Examples of a representative high strength copper alloy include a Cu—Be based alloy (for example, C17200; Cu—2% Be), a Cu—Ti based alloy (for example, C19900; Cu—3.2% Ti), and a Cu—Ni—Sn based alloy (for example, C72700; Cu—9% Ni-6% Sn). However, from the viewpoints of cost and environmental load, in recent years, a tendency to keep the Cu—Be based alloy at a respectful distance (so-called deberyllium orientation) has become strong. In addition, the Cu—Ti based alloy and the Cu—Ni—Sn based alloy have a modulated structure (spinodal structure) in which the solid solution elements have a periodic concentration fluctuation within a matrix and have high strength. However, there is involved such a drawback that the electrical conductivity is low as, for example, from about 10 to 15% IACS.
On the other hand, a Cu—Ni—Si alloy based (so-called Corson alloy) is watched as a material that is relatively excellent in a balance of properties between strength and electrical conductivity. For example, a Cu—Ni—Si based copper alloy sheet material can be adjusted to a 0.2% yield strength of 700 MPa or more while keeping a relatively high electrical conductivity (from 30 to 50% IACS) through steps on the basis of solution treatment, cold-rolling, aging treatment, finish cold-rolling, and low temperature annealing. However, in this alloy system, it is not always easy to respond to higher strength.
As a means for realizing high strength of the Cu—Ni—Si based copper alloy sheet material, general methods such as addition of large amounts of Ni and Si and increase of a finish rolling (temper rolling treatment) ratio after the aging treatment are known. The strength increases with an increase of the addition amounts of Ni and Si. However, when the addition amounts exceed a certain extent (for example, Ni: about 3%, Si: about 0.7%), the increase of the strength tends to be saturated, and it is extremely difficult to attain a 0.2% yield strength of 950 MPa or more. In addition, the excessive addition of Ni and Si easily brings a lowering of the electrical conductivity or a lowering of bending workability due to coarsening of a Ni—Si based precipitate. On the other hand, it is also possible to enhance the strength due to an increase of the finish rolling ratio after the aging treatment. However, when the finish rolling ratio increases, the bending workability, in particular, bending workability in “bad way bending” with the rolling direction as a warped axis is conspicuously deteriorated. For that reason, even when the strength level is high, there may be the case where the Cu—Ni—Si copper based alloy sheet material cannot be worked into an electrical or electronic part.